The present invention relates to radiofrequency (rf) electromagnetic (EM) field measurements and especially to such measurements made simultaneously and in the same location (same volume of space).
Measurements of rf complex, near field electromagnetic radiation sources, are frequently made to determine the intensity of the electric and magnetic fields at a given location in close proximity to RF sources, the reasons for such measurements being to ascertain electromagnetic radiation hazards to human beings.
At one time, field measurements were made with directive antennas but it was found that these antennas do not reliably measure complicated EM fields, such as those with reactive near-field components, multipath reflections, arbitrary polarization, multiple frequency components, complicated modulations and large field gradients. The accuracy of isotropic probes is independent of polarization of the measured radiation and direction of propagation. Those probes with square-law detection characteristics and broad bandwith overcome the uncertainties created by the complex EM fields because the outputs of three orthogonal antennas (either loops or dipoles) can be summed to obtain the square of the magnitude of the total field.
Previous EM field measurement systems have utilized two probes, one to measure the E-field and one to measure the H-field. In the near field of a radiation source, the EM field changes very sharply per unit volume, i.e., a very small change in spatial position of a three-dimensional probe can result in a large change in measured field intensity. Also, under typical usage situations, potentially hazardous fields generated by industrial or medical devices change rapidly with time (energy is generated for a few seconds per minute in a random pattern). If it is desired to measure, in the same spot, electric and magnetic field strengths which vary spatially and temporally, one must place two separate probes in exactly the same place at the same time, a physical impossibility. Even if the two probes are placed closely together, they cannot measure the respective fields at exactly the same point (volume) in space.
Alternatively, the probes may be placed at the same spot if used sequentially in time. However, the field under study may vary in intensity over the finite interval of time which elapses between performance of each measurement. Hence, measurement uncertainty exists in that the fields may vary rapidly temporally and spatially.